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Types of Marine Engines
A marine engine is one that powers a water craft. Unlike it's combustion counterpart that powers land vehicles, a marine engine is made for exclusive use in the water and has a few unique characteristics. Generally, there are two types of marine engines: inboards and outboards. An inboard engine is enclosed in the hull of a boat and is an outgrowth of a traditional car engine. An outboard motor is mounted outside the hull. To manoeuvre its craft, the engine pivots while the driveshaft turns the propeller.
Special Parts and Functions
Boat motors are made so parts don't rust or corrode because of extensive water exposure. Driveshafts, cam shafts, freeze plugs, manifolds and other parts are usually made of corrosion-resistant metals, such as brass or bronze. Alternators, starters and transmissions are also specially-constructed with metals able to withstand higher RPMs than traditional car engines. Valves are made to move much quicker than those on car engines. The amount of time intake and exhaust valves stay open is much shorter. This prevents water from leaking into the engine block. Marine motor carburettors also do not expel overflow fuel in to the atmosphere. Unlike a car carb, a marine unit reroutes overflow back into the carburettor. Special gaskets are used in marine motors to prevent water damage and keep gas and oil from being spewed into waters.
A marine engine works the same as a traditional car engine, in that fuel is burnt to move a driveshaft, which, in turn, sets the craft into motion. Marine engines are usually either two-stroke or four-stroke motors. Two-stroke engines perform two basic strokes of the cylinder. When the cylinder goes up, fuel ignites. On the down stroke, the motor fills with gas and exhaust is pushed out. A four-stroke engine burns fuel as pistons are moved up and down by the crankshaft and valves. Each time the crankshaft makes a revolution, it causes two stokes (one movement up and one down) of the pistons. This is done twice for a total of four strokes. Four-stroke engines burn straight petro-based gasoline. Oil lubricates parts from a separate tank and is not mixed with fuel. Diesel engines (two or 4-stroke) use heat compression to ignite fuel instead of spark plugs. This is done in a small combustion chamber. Diesel fuel is refined and burnt more efficiently than gasoline.
Marine engines are either cooled by closed cooling or raw water cooling systems. A closed cooling system uses antifreeze and water like a traditional combustion engine. Much like a radiator on a car, a closed system uses a heat exchanger, which regulates the temperature of water being circulated through the engine block. A raw water cooling system uses a circulation pump that allows the raw water the craft sits in to flow into the engine block to cool it.